Answer
(Fatwa: 1746/1366/L=1430)
Being the girl younger than boy is not necessary for marriage. It is also right for a boy to marry an older girl. Therefore, if really that girl is older than the boy, the Nikah is valid with her as well. The holy Prophet صلى الله عليه وسلم married first time with Hadhrat Khadija رضى الله تعالى عنهاwhen the holy Prophet Muhammad صلى الله عليه وسلم was 25 years old and Hadhrat Khadija رضى الله تعالى عنها was 40 years old.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband